Abstract

Freshwater mussels are physiologically adapted to living in freshwater habitats, contributing to water clarity and quality through their filter-feeding habit. Most of their larvae ('glochidia') are obligate parasites of fishes. Transformation to the juvenile stage is generally more successful on native and endemic host fishes with which they co-occur. Westralunio carteri was listed as 'Vulnerable' by the IUCN in 1995 presumably as a result of population decline due to salinisation. Recent field observations in the Swan-Canning catchments have shown that the species is sensitive to drought, elevated temperatures and hypoxia. Engineering works within Lower Helena Reservoir during April-May 2011 required draining a large area of the dam. Draining was discontinued when numerous W. carteri were observed stranded in the drying mud. Sampling of a total area of 120 m2 of exposed shoreline revealed a mortality of 25% while only 0.31% of mussels that remained submerged (400 m2) had died. Shell lengths of exposed mussels were significantly larger than submerged mussels (71 mm vs. 61 mm, P < 0.001). Lack of smaller individuals (<40 mm) and large numbers of more sizeable adults (>70 mm) suggests that the population within the dam has not had recent recruitment success. To protect the existing population from further loss, a portion of W. carteri (n = 1205) was relocated downstream into permanent pools for refuge.